Cytochromes P-450 (P-450) are key components of mixed function oxidases which activate many chemical carcinogens such as polyaromatic hydrocarbons, N-nitrosodimethylamine, and other aromatic amines. Some P-450s preferentially activate a certain chemical carcinogen, and identification and quantification of P-450 isozymes in tissues and organs would provide two keys to understanding the individual differences in sensitivities to chemical carcinogens. Our approach to this goal is the use of epitope specific monoclonal antibodies (MAbs) to P-450 isozymes. In addition to eight libraries of MAbs to different forms of P-450 (IAl in rats and fish, IA2 in rabbits, IIB1 in rats and rabbits, IIE1, IIC11 and IIIA1 in rats), two libraries of MAbs to P-450 IA2 and cytochrome b5 (b5) were established through the fusion of myeloma cells with spleen cells of mice which were immunized with P-450 IA2 and b5. MAbs to b5 were highly specific and did not cross-react with any P-450s or NADPH P-450 reductases which were tested. MAbs to b5 recognized rat liver microsomal b5 and also homologous b5 in human liver microsomes and in the homogenates of human TK- cells which were infected with vaccinia viruses carrying human b5 cDNA. P-45OIAl and P-45OIA2 were identified in the liver and lung of mice which were induced with 2,3,7,8-tetracholorodibenzo-P-dioxin and in the aorta endothelium of rats treated with 3-methylcholanthrene. P450 IIE1 related P-450s were identified in the liver and esophagus microsomes of rats and humans. Identification of specific P-450s with MAbs would be useful for determining the P-450 subtypes responsible for the activation of chemical carcinogens in the environment.